[TheForge] heavy metal math/c frame press

Tue Nov 25 14:17:01 2003

Andy,  First there is no such thing as NON-compressible fluid -- but I agree
that the compressibility of hydraulic oil and common system pressures are
low.  (2000 - 3000 psi).  Stored energy is no where near what it is in an
air or steam system  at low pressure such as 100 psi.  The compressibility
of the fluid in these systems along with the entrained air is of major
interest to those building high performance servo system using hydraulics.
The amount of compression and stored energy in very high pressure hydraulic
systems at 5000 to 10,000 psi is another story but for systems that
blacksmiths and bladesmiths use this would be unrealistic.  You point is
well taken -- for all practical purposes for the uses being made by
blacksmiths they can consider the fluid to be non-compressible.

The stored energy in a hydraulic press is another issue all together.  As a
press is loaded there is store energy in not only the oil but in the stretch
of the press itself.  If a fitting or hose fails at this point a rather
short duration high flow of oil can occur.  But this is still not the major
risk I am concerned about. -- The real danger is from a smaller leak that
causes a misting flow of the hydraulic oil -- which that comes in contact
with the hot metal being worked and causes a fire / explosion of the mist
cloud.  Hydraulic oil in liquid form is quite safe but in mist form it is
much more like a gas.

The point of my referencing the pump failure that you noted is that it is
very rare and I have never seen one caused by the hydraulic pressure itself.
None the less in the industrial system that this pump fail there was a large
amount of stored energy because this system -- as do most high performance
industrial systems -- included major gas hydraulic accumulators to provide
high volume flow for short duration as required by the system.  Again -- 
this does NOT apply to the common simple systems that most blacksmith might
be building.  None the less I think it is important to not give the
impression that hydraulic system don't have cases of high stored energy.
Another example of high stored energy is when a load is support by a blocked
hydraulic cylinder such as in a fork lift or pay loader lift.  Failure to
understand this possible stored energy can result in serious injury or death
to those working on these type of systems.

I still want to come back and once again make the point that there is real
risk of fire / explosion between a hydraulic system and hot metal were a
small leak can create a mist that then can become a major fire ball.  Just
make sure your fittings, pipe, and hose are correctly rated for the
pressures you are using.  You don't want to spray / mist hydraulic oil on
hot metal !!

Dave Smucker
----- Original Message ----- 
From: "Andy Vida" <[email protected]>
To: <[email protected]>
Sent: Tuesday, November 25, 2003 12:47 PM
Subject: Re: [TheForge] heavy metal math/c frame press

>
>
> "David E. Smucker" wrote:
>
> > In 30 plus years I have seen one pump blow up... No fire no or other
> > damage just messy.
>
> Hydraulic oil is a non-compressible fluid, as is water.
> This means that zero energy is stored in its mass as
> it comes under pressure.  The only stored energy in
> the hydraulic system will be located in the conduits
> as they expand under load.  These are comparatively
> very rigid, therefore energy buildup in them is small
> in terms of volume.
>
> Once a line fails the pressure falls to zero virtually
> instantaneously. The capacity of the pump to maintain
> pressure across a drop (the point of failure) is a
> direct function of its volume output.  These pumps are
> by definition low volume/high pressure affairs, therefore
> their ability to maintain pressure across that drop is
> basically nonexistent.
>
> Also consider that the point of failure will continue to
> expand if a high load continues to be applied to it,
> requiring ever higher volume flow to maintain pressure.
> This constitutes in inherent failsafe system.
>
> If one's hand were to be situated directly on the point
> of failure of a 10KPSI line, I would say there was a
> definite chance for significant injury via poisoning
> (hydraulic fluids are toxic).  Otherwise, the pressure
> drop is so rapid that no significant danger is posed by
> the mechanical action of the fluid.  Shrapnel from a
> failing hard line is another story, but lines are designed
> specifically not to fail in that fashion.
>
> -Andy
> _______________________________________________
> http://mailman.qth.net/mailman/listinfo/theforge
> theforge mail list group photo site is
> http://www.photoaccess.com
> Login:  [email protected]
> password:  anvil
> ___________
>
>
>